<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="UTF-8">
  <title>Title</title>
  <style>
    input {
      color: #999;
    }
  </style>
</head>
<body>
<input type="text" name="" id="" value="手机">
<script>
  var input = document.querySelector('input');

  input.onfocus = function () {
    if(this.value === '手机'){
      this.value = '';
      this.style.color = 'black';
    }
  }
  input.onblur = function () {
    if (this.value === '') {
      this.value = '手机';
      this.style.color = '#999';
    }
  }
</script>
</body>
</html>